Primary cell



Patented Dec. 9, 1952 PRIMARY CELL Roy C. Kirk, Ashford B. Fry,

Midland, Mich., assignors to and Percy F. George, The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Application March 3, 1950,

Serial No. 147,568

Claims.

The invention relates to primary cell particularly those employing magnesium as the anode material and a carbon cathode depolarized with manganese dioxide.

In our copending applications Serial Nos. 73,063 and 73,064, filed January 27, 1949, issued as patents numbered 2,547,907 and 2,547,908, respectively, we have disclosed primary cells in which the active material of the anode is magnesium, the electrolyte is an aqueous solution of an alkali metal, alkaline earth metal or ammonium bromide, and the cathode is finely-divided carbon in admixture with a depolarizer of manganese dioxide.

We have now discovered that by including in the manganese dioxide-carbon cathode mixture of primary cells formulated as above-indicated one of the water-insoluble chromates of the group consisting of BaCrOr, PbCrO4, and ZnCrOr,

the capacity of the cells is increased. The invention then consists of the improved primary cell formulation hereinafter fully described and particularly pointed out in the claims.

Referring more specifically to the elements of the cell, the anode may be formed of either pure metallic magnesium, commercial magnesium or a magnesium-base alloy containing a major portion of the metal, all such materials being included by the term magnesium hereinafter employed. least 99.5 per cent purity is to be preferred, and impurities such as iron, copper, and nickel, which increase the corrodibility of the magnesium, are preferably to be kept as low as reasonably possible, preferably below 0.002 per cent in the case of iron, below 0.01 per cent in the case of copper, and below 0.001 per cent in the case of nickel. The usual structural magnesium alloys are generally suitable as anode material, particularly the magnesium-base alloys in which the magnesium content exceeds about 80 per cent and which contain the usual amounts of manganese and either or both aluminum and zinc. Preferred proportions of these alloying elements are aluminum 2 to 9 per cent, zinc 0.5 to 3 per cent, manganese 0.1 to 0.5 per cent. The presence in the alloy of 0.05 to 0.5 per cent of calcium is also desirable. The anode material may be in any of the forms in which magnesium articles may be produced, such as rod, sheet, plate, and the like.

The electrolyte comprises a water solution of one of the aforementioned bromides the concentration may range from about grams per liter to amounts producing near saturation. A generally useful range is from about 100 to 450 However, metallic magnesium of at grams per liter, 300 grams per liter being generally preferred.

For some purposes, as when the cells are to remain for long periods on open circuit or are to be used intermittently over long periods, it is desirable to reduce the rate at which the anode material is corroded by the electrolyte. This may be accomplished by including in the electrolyte a water-soluble chromic acid salt of a base of the alkali, alkaline earth, and ammonium bases, such as for example, in corrosion-inhibiting concentrations such as from about 0.01 to 10 gram per liter of solution. Preferable amounts are about 0.1 to 1.0 gram per liter.

The cathode comprises finely-divided carbon, such as carbon black, preferably acetylene black, which is intimately mixed with th finely-divided depolarizer of manganese dioxide and moistened with the electrolyte. An electrode of carbon or graphite in suitable form, .such as a rod, plate, or other shape is placed in direct contact with the cathode mixture as the cathode terminal of the cell. In the usual dry cell construction, this cathode terminal is a carbon rod embedded in the cathode mixture.

The water-insoluble chromates to be used in accordance with the invention in the aforesaid cathode mixture are generally available in finelydivided form and may be mixed with the cathode mixture without further comminution. To insure uniform distribution in the cathode mix, it is desirable to employ the insoluble chromate in a fineness of at least mesh. The insoluble chromate may be mixed either wet (with electrolyte solution) or dry with the manganese dioxide and comminuted carbon cathode material, although it is preferable to mix these ingredients in the dry condition. If mixed dry, the resulting mixture may be moistened to the desired extent with electrolyte.

The proportions of the ingredients in the cathode mixture, that is the manganese dioxide, carbon, and insoluble chromate, appear to be critical. There should be sufficient carbon in the mixture to render it adequately conductive, as for example about 3 to 15 per cent by weight, optimum amounts appear to be about 6 to 10 per cent on the dry basis; the insoluble chromate is to be present in amount between about 0.1 per cent and 10 per cent, a preferred amount is about 0.5 to .3 per cent; and the manganese dioxide 3 comprises the balance. In dry cell formulations, enough electrolyte solution is added to the cathode mix to render it moldable. Usually about 300 to 500 cc. of electrolyte solution is used per 1000 grams of the cathode mixture of carbon, manganese dioxide and water-insoluble chromate.

As illustrative of the effect on the capacity of cells formulated as described, tests were made in which D-size (flashlight size) dry cells were assembled using magnesium as the anode material, one of the aforesaid bromides dissolved in Water as the electrolyte and a mixture of manganese dioxide, acetylene black, and one of the aforesaid Water-insoluble chromates in accordance with the invention as the cathode mixture. For comparison, similar cells were formulated in similar manner but without including the waterinsoluble chromate in the cathode mix. In assembling the test cells, the anode was a D-size battery can extruded of a magnesium-base alloy having a nominal composition of 3 per cent aluminum, 1 per cent zinc, 0.3 per cent manganese, the balance being magnesium. Each can was lined with paper moistened with electrolyte, as in conventional cell construction. The cathode mix, moistened with electrolyte in the proportions of about 360 to 500 0.0. of electrolyte per 1000 grams of dry cathode mix, was tamped into the paper-lined can, a carbon rod inserted into the mix in the can, and the top of the can was sealed with wax leaving an air space between the wax seal and the top of the cathode mix in the can, also as in conventional cell construction. Test cells, thus assembled, were subjected to a discharge test to determine their capacity.

Prior to running the capacity test, each cell, as freshly made up, exhibits a voltage of 1.9 and this voltage is brought down to 1.7 volts by short circuiting each cell for 1.5 minutes. After the voltage is. thus reduced to 1.7, the capacity test is commenced and consists in continuously discharging the cell through a resistance of 7.5 ohms until the cell voltage, measured while thus discharging, declines to 0.9 volt. The number of hours of such discharging is taken as the measure of the capacity.

The results of such tests of representative examples of cells formulated in accordance with the invention, and others for comparison, are set forth in the accompanying table.

4 As set forth in the table, the second and third columns show the proportions in grams per liter (g. p. l.) and kinds of salts dissolved in water to form the electroylte. The next three columns 5 show the proportions of the manganese dioxide, acetylene black (A. B.), and water-insoluble chromate, if any, in per cent by Weight on the dry bases in the cathode mixture. The last column records the average number of hours of continuous discharge through 7.5 ohms of two cells of the same formulation before the voltage of each cell declines to 0.9 volt.

Referring to the table, it will be observed that cells having the formulations numbered 1, 3, 7, 11, and 17 do not contain an insoluble chromate in the cathode mixture and are included in the table for comparative purposes. The increase in capacity obtained by including the insoluble chromate in the cathode mixture is illustrated by the remaining formulations and depends at least in part upon the composition of the electrolyte, the greatest increases being obtained with lead chromate in cells employing ammonium bromide electrolyte. For example, the cells of formulation 18 show 25.5 per cent more capacity than the comparison cells of formulation 17. The addition of lead chromate to the cathode mixture of cells operated on magnesium bromide electrolyte increases their capacity as much as 20.5 per cent as shown by comparing the capacity of cells of formulation number 7 with those of formulation number 8. Barium chromate increases the capacity of cells operating with magnesium bromide as electrolyte as much as 16 per cent as shown by comparing cells of formulations number 1 with those of formulation number 2. Barium chromate increases the capacity of cells having an ammonium bromide electrolyte to about the same extent as those having a magnesium bromide electrolyte, as shown by cells of formulation 19 compared to formulation 17 and by cells of formulation 9 compared to cells of formulation 7.

We claim:

1. In a primary cell having magnesium as the anode material, an aqueous electrolyte of a watersoluble inorganic bromide selected from the group consisting of the bromide of an alkali metal, alkaline earth metal, and ammonium, and a cathode comprising a mixture of finely-divided carbon and a depolarizer of manganese dioxide,

Table 1 7 Cathode Mix, Weight Percent lllectroly te Dry Average Formulation p l Hours,

g. p. 1. Bromide g. p. 1. Inhibitor M110 A. 13. to M l 300 MgBrzfiHzO. 0 1 LlzCIOl 9U 10 15. 6 d0 -d 88 10 2B3C1O4 18.1 94 6 23. 3 93 6 IPbCIO-l 2G. 0 92 B ZPbClOl 25. 5 91 6 3PbCrO4. 24. 3 9O 10 21. 9 87 1O 26. 4 87 1O 25. 3 87 1O 22. 9 9O 10 23. l 87 10 '24. 0 S7 10 26. 3 87 1O 24. 8 90 1O 27. 5 87 29. 1 on 26. 7 S7 10 3PbClO4 33. 5 S7 10 3BaCIO4 30. 9

1 Acetylene black.

the improvement which consists in including in the cathode mixture of water-insoluble chromate selected from the group consisting of barium chromate, lead chromate, and zinc chromate in the proportions of about 0.1 to per cent of the weight of the dry mixture, said dry mixture containing between about 3 and per cent by weight of the finely-divided carbon, the balance being manganese dioxide.

2. In a primary cell according to claim 1 in which the water-insoluble chromate is barium chromate.

3. In a primary cell according to claim 1 in which the water-insoluble chromate is lead chromate.

4. In a primary cell according to claim 1 in which the water-insoluble chromate is zinc chromate.

5. In a primary cell according to claim 1 in which the water-soluble bromide is magnesium bromide.

6. In a primary cell according to claim 1 in which the water-insoluble chromate is barium chromate and the electrolyte is magnesium bromide.

7. In a primary cell according to claim 1 in which the water-insoluble chromate is lead chromate and the electrolyte is magnesium bromide.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,521,295 Holler Dec. 30, 1924 1,696,873 Wood Dec. 25, 1928 2,343,194 Lawson Feb. 29, 1944 2,445,306 Lawson July 13, 1948 FOREIGN PATENTS Number Country Date 423,301 Great Britain Jan. 24, 1935 

1. IN A PRIMARY CELL HAVING MAGNESIUM AS THE ANODE MATERIAL, AN AQUEOUS ELECTROLYTE OF A WATERSOLUBLE INORGANIC BROMIDE SELECTED FROM THE GROUP CONSISTING OF THE BROMIDE OF AN ALKALI METAL, ALKALINE EARTH METAL, AND AMMONIUM, AND A CATHODE COMPRISING A MIXTURE OF FINELY-DIVIDED CARBON AND A DEPOLARIZER OF MANGANESE DIOXIDE, THE IMPROVEMENT WHICH CONSISTS IN INCLUDING IN THE CATHODE MIXTURE OF WATER-INSOLUBLE CHROMATE SELECTED FROM THE GROUP CONSISTING OF BARIUM CHROMATE, LEAD CHROMATE, AND ZINC CHROMATE IN THE PROPORTIONS OF ABOUT 0.1 TO 10 PER CENT OF THE WEIGHT OF THE DRY MIXTURE, SAID DRY MIXTURE CONTAINING BETWEEN ABOUT 3 AND 15 PER CENT BY WEIGHT OF THE FINELY-DIVIDED CARBON, THE BALANCE BEING MANGANESE DIOXIDE. 